What was unusual about the telescreen in Winston's apartment?

The telescreen is located in a position that does not command a view of Winston's entire room; in this way, Winston is able to write his diary in the little corner hidden from the screen. If this anomaly of construction hadn't existed, he would never have dared keep a diary, which he knows is an act forbidden by the Party.
Interestingly, we're told that what Winston is doing is not actually illegal. Nothing is really illegal because "there were no longer any laws." This is the apparent paradox embedded within a totalitarian society: nothing is explicitly prohibited, but everything a person does is suspect in the eyes of the regime, because its power is arbitrary. Another extraordinary concept in 1984, from the point of view of our own democratic (though still somewhat imperfectly so) society, is that "most of the proles did not even have telescreens in their homes." In one sense the telescreen, or at least part of its functionality, that Winston so fears and has to hide from is also, paradoxically, something we consider indispensable. To be without one would be like not having a smartphone, the absence of which would drive most of us crazy in our world of high-tech expectations. The proles, however, are both deprived of it (therefore living under "primitive" conditions) and simultaneously granted a degree of freedom Outer Party members such as Winston do not have. This accords with the Party's dictum that "the proles are not human beings." They are considered so inferior and beyond the pale that they do not need to be surveilled.
Orwell's implication is that technology will eventually be used as an instrument of oppression. When the ability to transmit and receive was combined in the same device, he tells us, "private life came to an end." One wonders if Winston could have been saved if the randomness of design in his old-style flat hadn't permitted him to keep a diary. Writing things down, expressing himself on paper, emboldens him to transgress further and further against the Party. Yet we're also told that the Thought Police can do just what is implied in their title: read people's minds—in which case Winston would have been doomed anyway.

To answer this question, take a look at Part One, Chapter One. First of all, you'll notice that it's not called a television. In Oceania, this television is, in fact, called a telescreen. Secondly, it does not have the same purpose as modern televisions. That is, it's not about providing news and entertainment to the viewers. In Winston's world, the telescreen transmits messages directly from the Party.
In addition, there is something else rather unusual about the telescreen, as Winston describes:

The instrument (the telescreen, it was called) could be dimmed, but there was no way of shutting it off completely. 

The telescreen cannot be turned off for an unusual, and rather sinister, reason: the Party uses it to monitor the every moment and conversation of Party members. It is like having a CCTV camera in Winston's apartment which live streams his every move to the Inner Party members.
The telescreen is, therefore, an important tool in keeping control of the people of Oceania, which explains its unusual functions.